Micrel’s MIC2569 is the only single-chip power controller that is fully compliant with the CableCard 2.0 specification that has been legally mandated for set-top boxes (STBs). The IC can be configured to support either single-stream (S-Mode) video cards or multiple-stream (M-Mode) video cards through a 3-bit digital parallel control interface (see Figure 1). It therefore promises to streamline power design for a range of platforms intended to support the FCC requirement that all STBs be CableCARD compliant as of July 2007, as well as the transition to digital television (which goes into full effect in the U.S. in February 2009; for more information, visit the FCC web site on this topic). These platforms include digital cable receivers, DTVs and HDTVs, DVR STBs, and digital satellite receivers.
The main electrical function performed by the device (shown in the Figure 2) is voltage switching at its three voltage outputs, VCC, VPP1 and VPP2. The 3.3-V rail is switched through VCC, and both VPP1 and VPP2 can switch between 3.3 V and 5.0 V.
Alex Baskind, Mixed Signal Product Marketing Manager at Micrel, states that the 5.0-V supply is not a fundamental requirement for all CableCARD operating modes, being mainly used for low-power legacy devices or to support non-volatile memory, such as Flash. However, 3.3 V is a fundamental operating requirement, and is also used to power the circuitry of the MIC2569.
All voltage switching is soft-start at turn-on, and break-before-make sequencing is applied when an output is changed between the 5.0-V and 3.3-V voltage supplies. Current limiting built into the MIC2569 protects both VCC and VPP output lines of the host system from card faults and accidental short circuits. Furthermore, external output-filter capacitors tied to the IC can be discharged through dedicated internal shunting FETs.
The device architecture that performs the dual-voltage switching is a complimentary pair of FETs, with the source of the PMOS device tied to the 5.0-V rail, and the drain of the NMOS tied to the 3.3-V rail. Baskind states that a charge pump is used to boost the NMOS gate voltage to prevent the gate-source voltage of the NMOS from reducing the voltage available at the drain. Each complimentary pair also uses a special circuit architecture to prevent forward biasing in the body diode of the NMOS when the NMOS is off and the PMOS is on.
The circuit design includes additional features that provide added protection against internal and external events, such as thermal stress or ESD. For example, according to Baskind, each output of the IC includes a dedicated power-stage thermal sensor. This improves the sensitivity and response time needed for the IC to detect and correct an internal overtemperature condition caused by a sustained overcurrent condition.
Additionally, the IC provides an output fault signal to communicate the occurrence of a fault condition to the host.
Baskind also states that while there are no PCMCIA/CableCARD specifications for ESD, Micrel uses a 2-kV human-body model, and a 500-V machine model to improve the reliability of the IC. Furthermore, CableCARDS do not directly interface to ac line voltages or RF front ends, making them essentially immune to ESD damage. However, given the anticipated high volumes for the target application for the MIC2569, ESD robustness is considered essential for the design.